Liquid crystal display (LCD) devices are display devices which control transmission/blocking of light (on/off of image display) by controlling the alignment of liquid crystal molecules having birefringence properties. For alignment of liquid crystal molecules, for example, an alignment film with aligning ability is formed on a face to contact to a liquid crystal layer of a substrate. The aligning ability is provided by alignment treatment such as rubbing method or photo-alignment method.
Another example of the method for controlling the alignment of liquid crystal molecules is multi-domain vertical alignment (MVA) mode, in which the alignment of liquid crystal molecules is controlled by structures for alignment control, such as dielectric protrusions provided on an electrode or slits in an electrode, without performing an alignment treatment. In MVA mode, the structure for alignment control enables to align liquid crystals in a plurality of different directions while a voltage is applied to the liquid crystals, even without rubbing treatment on an alignment film. Thus, MVA mode achieves better viewing angle characteristic than conventional TN mode.
However, the regions with protrusions or slits are likely to have low light transmittance. The regions are allowed to have higher light transmittance by simplifying the arrangement and increasing the distances between the protrusions or the spaces between the slits. When the distances between the protrusions or the spaces between the slits are too large, however, the propagation of the tilting of liquid crystal molecules takes a longtime. Thus, the liquid crystal molecules respond very slowly to application of a voltage necessary for image display to the liquid crystal layer.
In order to increase the response rate, studies have been made on a technology of forming, on an alignment film, a polymer layer on which the tilt direction of liquid crystal molecules is recorded (hereinafter also referred to as polymer sustained alignment (PSA) technology). The tilt direction is recorded by injecting a liquid crystal composition containing a polymerizable monomer between substrates, followed by polymerization of the monomer under voltage application (see, for example, Patent Literature 1).
Moreover, vertical alignment-twisted nematic (VA-TN) mode using a photo-alignment film is known as an effective method to improve both the light transmittance and the response rate. The photo-alignment film is an alignment film which exhibits an alignment-control capability for controlling the alignment of liquid crystal molecules in its entire region exposed to light. Thus, unlike MVA mode, formation of protrusions or slits for controlling the alignment of liquid crystal molecules is not necessary, which leads to an improvement in the light transmittance. Since the alignment film has the alignment-control capability over its entire region, all liquid crystal molecules facing the alignment film are concurrently aligned, unlike MVA mode which allows liquid crystal molecules to align in a domino-like manner. Thus, the response rate is increased.
Furthermore, application of twisted-nematic (TN) mode, specifically a technique to provide a plurality of domains having different alignment directions from one another in a region corresponding to one pixel in the liquid crystal layer, enables to achieve excellent viewing angle characteristic at the same time. The number of the domains is preferably 4, which enables to achieve well-balanced viewing angle characteristic.
However, in the case of a liquid crystal mode using a conventional PSA technology or a photo-alignment film, image sticking sometimes occurs in a liquid crystal display. Image sticking is a phenomenon where, when an image is displayed for a long time and the image is changed with a different image, the previously displayed image remains visible on the screen.
The inventors of the present application have found out that one of the causes of image sticking is a phenomenon (hereinafter, also referred to as Δtilt) where the tilt of liquid crystal molecules is not fully returned after continuous application of a voltage to the liquid crystal molecules, or in other words, changes occur in the tilt angle from that in the initial state. FIG. 13 and FIG. 14 show conceptual images illustrating the principle of image sticking caused by insufficient returning of the tilt of liquid crystal molecules. FIG. 13 illustrates the state where a voltage is applied, whereas FIG. 14 illustrates the state when the voltage is changed to a voltage for halftone display after application of a voltage for a certain period of time. Meanwhile, in the examples explained above, liquid crystal molecules having a negative dielectric constant anisotropy are used.
As shown in FIG. 13, a liquid crystal display panel typically includes a pair of substrates 111 and 112 each having a polarizing plate applied thereto, and a liquid crystal layer 113 disposed between the pair of substrates 111 and 112. The liquid crystal layer 113 contains a plurality of liquid crystal molecules 151 which align differently depending on the level of voltages applied inside the liquid crystal layer 113. In the example shown in FIG. 13, regions in both ends are regions of black display where no voltage is applied. In this state, the liquid crystal molecules 151 align in a substantially perpendicular direction to the surfaces of the substrates 111 and 112. Thus, light having passed through one of the polarizing plates penetrates the liquid crystal layer 113, and is then blocked by the other polarizing plate. A region in the middle is a region of white display where a threshold or higher voltage is applied. In this state, the liquid crystal molecules 151 tilt in a substantially horizontal direction to the surfaces of the substrates 111 and 112. Thus, light having passed through one of the polarizing plates penetrates the liquid crystal layer, and then further passes through the other polarizing plate to exit as display light.
In contrast, when the voltage is changed to a voltage for halftone display as shown in FIG. 14, the liquid crystal molecules 151 in the regions of black display tilt diagonally to the surfaces of the substrates 111 and 112 to provide a desired halftone display. However, the liquid crystal molecules 151 in the region of long-time white display may not fully return to the desired diagonal tilt direction to the surfaces of the substrates. As a result, images in the region are displayed brighter (occurrence of image sticking) than those in other halftone display regions.
In order to prevent the Δtilt of liquid crystal molecules in a liquid crystal mode using a photo-alignment film, studies have been made on a technology of forming, on an alignment film, a polymer layer on which the tilt direction of liquid crystal molecules is recorded. The polymer layer is formed by injecting a liquid crystal composition containing a polymerizable monomer between a pair of substrates, at least one of the pair of substrates being provided with a photo-alignment film, followed by polymerization of the monomer without voltage application (see, for example, Patent Literature 2).